Intensity could play another factor too, remember in X2 when Wolverine stabs the soldier to the fridge, that had to be cut a bit because of how intense it came off, and there wasn't a drop of blood in that scene.

And I had a friend (who's a huge X-Men fan), who didn't let his young son see X2 because of that mansion invasion scene. It didn't matter that it wasn't bloody, he said Wolverine was obviously killing people and he thought it was too much for his kid. He loved it...he just saw it sans kid at the time.

__________________”We live in times when hate and fear seem stronger. We rise and fall, and light from dying embers: remembrances that hope and love last longer. And love is love is love is love is love is love is love cannot be killed or swept aside."